Why don't some of my URLs get prefixed with DocumentRoot when using mod_rewrite?

Answer:

If the rule starts with /somedir/... make sure that really no /somedir exists on the filesystem if you don't want to lead the URL to match this directory, i.e., there must be no root directory named somedir on the filesystem. Because if there is such a directory, the URL will not get prefixed with DocumentRoot. This behaviour looks ugly, but is really important for some other aspects of URL rewriting.